1. Field of the Invention
A process for reductively stripping an organic phase containing either or both uranium and vanadium to recover either or both the uranium and vanadium therefrom.
2. Description of the Prior Art
Phosphoric acid is generally produced commercially by one of two methods. One method is generally called the "furnace" method, and the other is generally referred to as the "wet process" method. In the wet process method of producing phosphoric acid, phosphate rock is contacted with a mineral acid such as sulfuric acid. Most phosphate rock contains metal compounds in varying amounts. In many cases, these metal compounds are dissolved from the phosphate rock and appear in the wet process acid as contaminants. Uranium and vanadium compounds are among those dissolved from the phosphate rock. A prominent method for separating and recovering either or both the uranium and vanadium compounds from the wet process acid has involved solvent extraction employing selected organics as extractants.
One such process for the separation and recovery of uranium from wet process phosphoric acid is disclosed by U.S. Pat. No. 3,711,591. The extraction of the uranium from the wet process acid is effected by contacting the wet process phosphoric acid with an organic phase comprising di-2-ethylhexyl phosphoric acid (D2EHPA) and trioctylphosphine oxide (TOPO) dissolved in an organic diluent. The extracted uranium is separated from the organic phase by contacting with a reductive strip solution comprising ferrous iron dissolved in phosphoric acid solution (wet process acid) to form a concentrated uranium-containing solution. The process then calls for the reoxidation of the uranium to the hexavalent state and reextraction in a second extraction cycle with the same extractant. The higher concentration of uranium in the wet process phosphoric acid solution enables a more efficient extraction with the organic phase. The uranium-loaded organic phase then is stripped with a carbonate solution to form a uranium precipitate that is recovered.
The addition of iron to the wet process phosphoric acid to form the ferrous iron-containing stripping solution presents several problems. First, iron addition to the wet process acid causes sludge formation during the subsequent processing of the wet process phosphoric acid to form phosphate fertilizer. Second, the iron addition may cause iron phosphate solids to form within the solvent extraction apparatus. Both of these problems result in phosphate loss and hence lower fertilizer yield. A third disadvantage is the limited solubility of iron in the wet process phosphoric acid. The reducing power of the iron-phosphoric acid solution is more effective as the ferrous iron concentration is increased. Unfortunately, as the ferrous iron concentration is increased, the amount of iron phosphate precipitate increases and may contaminate the solvent extraction process.
One process for the separation and recovery of vanadium from wet process phosphoric acid is disclosed by U.S. Pat. No. 3,700,415, which uses an organic extractant comprising a neutral organophosphorus complexing agent in a water-immiscible organic diluent. The organic phase is introduced into a mixer-settler wherein it is subjected to washing to remove co-extracted phosphoric acid to reduce the quantity of alkaline stripping agent required to strip the vanadium from the organic phase. Alternatively, inorganic salt solutions are used to scrub the organic phase. The scrubbed organic then is stripped by contacting with an alkaline stripping agent.
One process for the simultaneous separation and recovery of uranium and vanadium is disclosed by U.S. Pat. No. 3,836,476, assigned to the same assignee as the present invention. That process comprises contacting the wet process acid with an organic extractant comprising a dialkyl phosphoric acid, a trialkylphosphine oxide compound and an organic diluent whereby the uranium and vanadium are coextracted into the organic phase. The loaded organic phase is separated from the wet process phosphoric acid and the uranium and vanadium are stripped therefrom with an aqueous base or other alkaline stripping agent such as ammonium hydroxide or sodium carbonate.
It would be desirable to provide a reductive stripping process whereby the recovered uranium and vanadium is not contaminated by undesirable alkali metal ions and is recovered in a concentrated form with the use of a minimum of reductant.